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Close and far moons in 2018

In 2018, the moon swings farthest from Earth for the year on January 15, only 2 weeks after coming closest to Earth for the year on January 1.

Here’s a comparison between the December 3, 2017 full moon at perigree (closest to Earth for the month) and 2017’s farthest full moon in June at apogee (farthest from Earth for the month) by Muzamir Mazlan at Telok Kemang Observatory, Port Dickson, Malaysia.

The moon’s distance from Earth varies throughout its monthly orbit because the moon’s orbit isn’t perfectly circular. Every month, the moon’s eccentric orbit carries it to apogee – its most distant point from Earth – and then to perigee – the moon’s closest point to Earth – roughly two weeks later.

In this post, beneath the illustration below, we list the year’s 14 perigees and 13 apogees. Yes, the moon’s apparent size in our sky does change across this cycle of the moon. The variation in the moon’s apparent size – across its monthly orbit – is akin to that of a U.S. quarter versus a U.S. nickel.

Also in this post, we share with you a little-known fact about the intriguing cycle of close and far moons.

This year’s closest perigee comes on January 1, 2018 (221,559 miles or 356,565 km) and the farthest apogee happens on January 15, 2018 (252,565 miles or 406,464 km). That’s a difference of about 30,000 miles (50,000 km). Meanwhile, the moon’s mean distance (semi-major axis) from Earth is 238,855 miles (384,400 km).

The moon’s orbit around Earth isn’t a circle, but it’s very nearly circular, as the above diagram shows. Diagram by Brian Koberlein. Used with permission.

Amazingly, in periods of four years, lunar apogees and perigees fall on the same, or nearly the same calendar dates. Let’s look four years ahead, to the year 2022:

Lunar perigees and apogees in 2022

Perigee

Apogee

Jan 1

Jan 14

Jan 30

Feb 11

Feb 26

Mar 10

Mar 23

Apr 7

Apr 19

May 11

May 17

June 2

June 14

June 29

July 13

July 26

Aug 10

Aug 22

Sept 7

Sept 19

Oct 4

Oct 17

Oct 29

Nov 14

Nov 26

Dec 12

Dec 24

Also, in cycles of two years, the calendar dates remain the same, or nearly so, except that the lunar apogees and perigees trade places. For instance, let’s look two years beyond 2018, to the year 2020:

Here’s a little-known fact of the moon’s apogee/perigee cycle, among both professional astronomers and lay people. That is, the cycle causes lunar apogees and perigees to align on the same, or nearly the same, calendar dates every four years. That’s because 53 returns to perigee (or apogee) are nearly commensurate with four calendar years.

The mean length of the anomalistic month (perigee to perigee, or apogee to apogee) is 27.55455 days, whereas the average Gregorian year equals 365.2425 days. Hence:

27.55455 x 53 = 1460.3912 days

365.2425 x 4 = 1460.97 days

This animation by Peter Lowenstein in Zimbabwe contrasts the size of the May 27, 2017 waxing crescent moon, which was close to Earth, with the June 9, 2017 full moon, which was far from Earth. Read more about this image.

Bottom line: In periods of four years, lunar apogees and perigees fall on the same, or nearly the same calendar dates.

Bruce McClure has served as lead writer for EarthSky's popular Tonight pages since 2004. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.